1. Field of the Invention
The present invention is related to a three dimensional image analyzing process. Particularly, the present invention is related to an apparatus and a method for performing a process to detect abdominal cavity regions of subjects within three dimensional images, and to a program that causes a computer to execute the method.
2. Description of the Related Art
Amounts of body fat and body fat percentages are measured by detecting regions corresponding to CT values that represent fat within cross sectional images perpendicular to the axes of the bodies of subjects (axial cross sections) in the vicinity of the spleens of the subjects, imaged and obtained by CT. A technique has been proposed that separates the fat regions within the images into subcutaneous fat regions and visceral fat regions by an image analyzing process.
Specifically, methods have been proposed, in which an outline is tracked along the outer periphery of the surface of a subject's body within axial tomographic images obtained by CT, regions of interest on the surface of the body are extracted, and total fat regions are extracted from within the regions of interest on the surface of the body. Then, the total fat regions within the regions of interest on the surface of the body are removed, regions corresponding to the abdominal wall muscle layer are extracted, and the outlines of the abdominal wall muscles are tracked, to extract visceral regions of interest that surround visceral fat. Thereafter, the visceral fat regions are extracted from within the visceral regions of interest, and subcutaneous fat regions are extracted by subtracting the visceral fat regions from the total fat regions (refer to Patent Documents 1 and 2, for example). Generally, there are gaps within the abdominal wall muscle layer. These methods take these gaps into consideration when tracking the outlines of the abdominal wall muscles, and the outlines are tracked by rolling a small circle such that it contacts the outer periphery of the abdominal wall muscle layer.
In addition, another method for discriminating visceral fat regions from subcutaneous fat regions has been proposed (refer to Patent Document 3, for example). In this method, axial tomographic images of the abdomen obtained by CT are classified into skin/muscle pixel regions, fat pixel regions, and subject exterior regions. The skin/muscle pixel regions adjacent to the subject exterior pixel regions are replaced with the subject exterior pixel regions to remove the skin/muscle pixel regions that represent the skin on the surface of the body. Then, the skin/muscle pixel regions that remain after removal are expanded such that the skin/muscle pixel regions that represent the abdominal wall muscle and the peritoneal membrane completely surround the fat pixel regions that correspond to visceral fat. Thereafter, the fat pixel regions adjacent to the subject exterior pixel regions and components linked to these pixel regions are labeled. The labeled regions are expanded toward the interior for an amount corresponding to the amount of expansion of the aforementioned skin/muscle pixel regions, and designated as subcutaneous fat regions. The fat pixel regions within the images other than the subcutaneous fat regions are designated as visceral fat. Further, Patent Document 3 discloses that the aforementioned method may be performed with respect to a plurality of axial tomographic images of the abdomen, the area of the subcutaneous fat region and the visceral fat region of each of the each image may be multiplied by the slice thickness of the image, and the sum of the multiplication results for each image may be calculated, to derive the volume of the subcutaneous fat and the visceral fat within the entirety of the abdomen.    Patent Document 1:    Japanese Unexamined Patent Publication No. 2002-222410    Patent Document 2:    Japanese Unexamined Patent Publication No. 2004-057275    Patent Document 3:    Japanese Unexamined Patent Publication No. 2003-339694
In the case that the axial image illustrated in FIG. 14A is employed to measure subcutaneous fat and visceral fat, for example, a fat region within a region Rgn1 within the abdominal cavity covered by the peritoneal membrane should be designated as visceral fat, and a fat region within a region Rgn2 between the surface of the body and the outer periphery of the abdominal wall muscle should be designated as subcutaneous fat. However, a fat region within a region Rgn3 between the outline of the abdominal cavity and the outer periphery of the abdominal wall muscle in the vicinity of the area behind the spinal column should not be designated as either subcutaneous fat or visceral fat. FIG. 14B illustrates examples of a visceral fat region Rgn1f and a subcutaneous fat region Rgn2f to be extracted at the vicinity of the upper edge of the abdominal cavity, FIG. 14C illustrates examples of a visceral fat region Rgn1f and a subcutaneous fat region Rgn2f to be extracted at the vicinity of the center of the abdominal cavity, and FIG. 14D illustrates examples of a visceral fat region Rgn1f and a subcutaneous fat region Rgn2f to be extracted at the vicinity of the lower edge of the abdominal cavity.
However, in the methods disclosed in the aforementioned patent documents, the outer periphery of the abdominal wall muscle to the exterior of the peritoneal membrane is used as a reference, and all fat regions toward the interior thereof are judged to be visceral fat. Therefore, the fat regions within the regions Rgn3 of FIG. 14 will be erroneously judged to be visceral fat. Fat regions are often observed in the vicinities of areas behind the spinal column in axial tomographic images in the vicinity of the pelvis. Therefore, if the volume of body fat is measured three dimensionally by calculating the sum of measurement results obtained from a plurality of axial tomographic images of the abdominal region, the erroneous judgments will cause the amount of errors to become great.
Meanwhile, in the case that body fat is measured using only axial tomographic images in the vicinity of a subject's spleen, the states of structures within the subject included in the images will vary due to the influence of the subject's respiration and the like. Therefore, it is not necessarily the case that accurate measurement can be performed. Accordingly, the method for measuring body fat three dimensionally by calculating the sum of measurement results obtained from a plurality of axial tomographic images as disclosed in Patent Document 3 is effective for such cases.
However, the method that defines the boundaries between subcutaneous fat and visceral fat by expanding the skin/muscle pixel regions that represent the abdominal wall muscle and the peritoneal membrane disclosed in Patent Document 3 assumes that the visceral fat region is surrounded by the abdominal wall muscle. Therefore, although a certain degree of accuracy in definition can be expected in axial tomographic images in the vicinity of the spleen, the assumption fails in axial tomographic images in the vicinity of the pelvis. Accordingly, the boundaries between subcutaneous fat and visceral fat cannot be accurately defined in these images.
As described above, there is demand to more accurately detect the abdominal cavity regions of subjects, in order to discriminate between subcutaneous fat regions and visceral fat regions, to more accurately measure the body fat of subjects.
The present invention has been developed in view of the foregoing circumstances. It is an object of the preset invention to provide an apparatus, method, and a program for detecting three dimensional abdominal cavity regions that realize more highly accurate three dimensional detection of the abdominal cavity regions of subjects.